"Now I'm using a WSM and remember when I tried fat side up on my old drum smoker I made I thought it was more tender but dryer tasting on the bark or what was left of it after the seasoning fell off and grill marks."
I cook fat side up on the WSM and always have the water pan in place (with or without water) as a shield. The grill marks aren't really grill marks (my experience). It is just an indentation of the grates from the weight of the meat and not noticeable after slicing. Also, the cap holds moisture in. So, if evaporation moves up away from the meat, the cap should help hold it in a bit (when cap up). Little more juicy? Maybe.
"I'm not convinced the fat renders so much it goes into the meat that deep"
Agreed. The fat will not render into the meat. Fat=oil and meat=water. It will not penetrate the meat. Maybe baste the sides slightly?
"He also slices his briskets fat side up....never occurred to me to try that. Much difference for those that do?"
Never tried. I like to see where I'm slicing.
"He also uses mustard for slather. No on this one for me. I think it creates too much of a burnt taste and think it's harder for the rub to penetrate the meat. So I stopped using mustard or oil for those reasons."
Some swear by it. To help hold the rub on or to get the little bit of tenderizing effect from mustard. Never used oil and can't say I noticed enough of a difference when I did use mustard.
So, with all this said. Personal preference
I cook fat side up on the WSM and always have the water pan in place (with or without water) as a shield. The grill marks aren't really grill marks (my experience). It is just an indentation of the grates from the weight of the meat and not noticeable after slicing. Also, the cap holds moisture in. So, if evaporation moves up away from the meat, the cap should help hold it in a bit (when cap up). Little more juicy? Maybe.
"I'm not convinced the fat renders so much it goes into the meat that deep"
Agreed. The fat will not render into the meat. Fat=oil and meat=water. It will not penetrate the meat. Maybe baste the sides slightly?
"He also slices his briskets fat side up....never occurred to me to try that. Much difference for those that do?"
Never tried. I like to see where I'm slicing.
"He also uses mustard for slather. No on this one for me. I think it creates too much of a burnt taste and think it's harder for the rub to penetrate the meat. So I stopped using mustard or oil for those reasons."
Some swear by it. To help hold the rub on or to get the little bit of tenderizing effect from mustard. Never used oil and can't say I noticed enough of a difference when I did use mustard.
So, with all this said. Personal preference